Methylphosphonyl dichloride is useful as an intermediate in the preparation of dimethyl pentaerythritol diphosphonate which is in turn useful as a flame-retardant. U.S. Pat. No. 4,154,721 (Valdiserri et al.) shows the use of this compound in combination with a wide variety of halogenated organic compounds to impart a high degree of flame retardancy to polypropylene, ABS resins and polystyrene. Its preparation can be accomplished by reaction of the methyl phosphonyl dichloride of this invention with pentaerythritol in accordance with the equation: ##STR2##
The preparation of phosphonyl dichlorides has been accomplished in the past by a number of different processes none of which have proved to be economically satisfactory. These are shown in Kosolapoff's "Organophosphorus Compounds", New York, Wiley (1950), at pages 61-63. Thus, the action of phosphorus pentachloride on the corresponding phosphonic acid yields the phosphonyl dichloride. ##STR3##
Another process, applicable to the preparation of phenylphosphonyl dichloride, involves the reaction of acetic acid and phenyl phosphorus tetrachloride. ##STR4##
Esters of phosphonic acid likewise can be made to yield phosphonyl chlorides, by reaction with phosphorus pentachloride. ##STR5##
Still another method has involved oxidation of phenyldichlorophosphine (C.sub.6 H.sub.5 PCl.sub.2) with oxygen (or air). Unfortunately, this method is somewhat hazardous unless all traces of free phosphorus are absent, and rather drastic conditions of time and temperature are required. Moreover, yields are low.
U.S. Pat. No. 3,188,281 (Briggeman et al.) shows the preparation of methylphosphonyl dichloride ("methyl dichlorophosphine oxide") by the phosgenation (with phosgene) of "pyro mix", a complex reaction mixture obtained in turn from the reaction of methanol and phosphorus trichloride.
All of the above methods of preparation, however, are fraught with disadvantages in the form of poor yields, inconvenient reaction conditions, etc. No convenient method for the preparation of this simple compound in good yields seems to have been available.
U.S. Pat. No. 2,683,168 (Jensen et al.) shows the reaction of an organic compound with phosphorus trichloride and oxygen to form a phosphonyl dichloride. The organic compound must contain an aliphatic carbon atom which is bonded only to carbon and hydrogen atoms and it must be free of sulfur and selenium. Such organic compounds can include hydrocarbons, chlorinated hydrocarbons, ethers, esters, ketones, etc., but apparently not methane or ethane.
U.S. Pat. No. 3,210,418 (Pianfetti) shows the preparation of aliphatic dichlorophosphines such as methyl dichlorophosphine by the reaction of methane and phosphorus trichloride. The reaction is catalyzed by a homogeneous, gaseous catalyst such as oxygen, a halogen, nitrogen oxides, etc. The reaction is carried out at high temperatures, e.g., 575.degree. C.
"Formation of Phosphonous Dichlorides by Alkylation of Phosphorus Trichloride with Methane or Ethane" by Pianfetti et al., J. Am. Chem. Soc., 84, 851-854 (1962), shows much the same subject matter as the above Pianfetti patent.